The origin of the name peridot is uncertain. The Oxford English Dictionary suggests an alteration of Anglo Norman pedoret (classical Latin paederot), a kind of opal, rather than the Arabic word faridat, meaning "gem". Peridot is one of the few gemstones that occur in only one colour, an olive green. The intensity and tint of the green, however, depends on how much iron is contained in the crystal structure, so the colour of individual peridot gems can vary from yellow to olive, to brownish-green. The most valued colour is a dark olive-green. Olivine, of which peridot is a type, is a common mineral in mafic and ultramafic rocks, and it is often found in lavas and in peridotite xenoliths of the mantle, which lavas carry to the surface; but gem quality peridot only occurs in a fraction of these settings. Peridot can be also found in meteorites. It is sometimes mistaken for emeralds and other green gems.